The present invention relates to a metal operating element for operating the valves of an internal combustion engine, the element having an insert of ceramic material in its surface in contact with the cams of a camshaft. The term operating element, as used in the description and claims of the present patent application, is to be understood to include valve lifters, rocker arms, pull levers, valve rockers and tappets.
Operating elements of the above-mentioned kind, especially tappets, are disclosed in German Federal publication Nos. DE-A 32 05 878, 32 39 325 and 33 32 455. As methods for fastening the inserts, publication No. DE-A 32 39 325 discloses shrink-fitting, clamping, cementing and casting in aluminum, while publication Nos. DE-A 32 05 878 and No. DE-A 33 32 455 call for cementing with epoxy resin. The proposal has been disclosed in practice that ceramic inserts be fastened to the metal operating elements by brazing. All of these proposals have certain disadvantages, so that in spite of the outstanding properties of the ceramic materials operating elements having an insert of ceramic material have not been widely accepted.
In detail, the disclosed proposals have the following disadvantages:
Cemented bonds which have sufficient and lasting strength in contact with water containing lubricants, even up to temperatures of 150.degree. C., provide insufficient strength under the conditions to which they are exposed in an engine. Epoxy resin bonds which of themselves (not as measured in the working environment) have shear strengths of 20 to 40 Mpa, have proven to be insufficiently reliable. Under the effect of oil and water a strength loss amounting to as much as 50% has been observed.
Brazed joints between ceramic inserts and metal operating elements, which have a still greater shear strength of about 100 MPa can be used only if the coefficients of expansion of the metal and of the ceramic insert are close to one another. If, however, there is a great difference in the coefficients of expansion, tensions are produced both in the ceramic insert and in the brazed junction which lead to cracking and spalling. Another disadvantage of brazed joints lies in the high temperatures of 800.degree. to 900.degree. C. which are necessary for their production. Areas of the metal operating element that have already been heat treated can be degraded thereby. Also disadvantageous are the need to perform the brazing in a vacuum or under shielding gas, the high energy consumption necessitated by the high temperatures, and the high cost of the silver-containing brazing materials.
The embedding of ceramic inserts by casting in metal is possible only if the operating element is made of aluminum, because as a result of the high casting temperatures required with other metals and the resultant thermal shock applied to the ceramic inserts there is a danger of cracking them. Since, however, operating elements of cast iron or steel are necessary for reasons of strength, this mounting technique cannot be practiced in many areas of engine construction. For it has been found that inserts cast in aluminum are not held sufficiently tightly under high stress. The same disadvantages have also been observed in the case of ceramic inserts which were fastened only by shrink fitting or by clamping in the metal operating elements.
Setting out from the disadvantages of the known operating elements, it is the object of the present invention, in an operating element of the above-described kind, to improve the strength of the bond to the ceramic insert such that sufficient strength will be assured even under great and long-lasting stress. Furthermore, the invention is intended to increase the possible combinations of metal and ceramic components, and to create the economic conditions essential to series production by devising a very simple and troublefree method of producing them.